FIG. 1 is a plan view illustrating a specific example of a conventional lead frame for use in a plastic package type semiconductor device.
The lead frame is repeatedly formed in plural number in an elongated band-shaped member in the longitudinal direction. FIG. 1 illustrates one of the lead frame portions 10. At a substantially central portion of a square frame portion 5, a square island portion 3 supported by a suspension lead 7 is formed, and about the island portion, a plurality of lead members 1 extending from the frame portion 5 toward the island portion 3 and a tie bar 8 for supporting adjacent lead members 1 each at a certain interval are formed. The lead member 1, with respect to the tie bar 8, comprises an internal portion (inner lead portion) 4, which is close to the island portion 3, and an outer portion (outer lead portion) 6, which is close to the frame portion 5, the tip end of the inner lead portion 4 extending to a position spaced apart from the island portion 3 with a predetermined distance.
In order to manufacture the plastic package type semiconductor device utilizing the lead frame, first, as illustrated in FIGS. 2 and 3, a semiconductor chip 2 is adhered on the island portion 3 of the lead frame 10 by means of an adhesive agent to interconnect each electrode pad of the semiconductor chip and the tip end portion of the inner lead portion 4 of the lead member 1 by means of a fine metallic line (bonding wire) 26. Next, the lead frame is attached in position within a mold.
FIG. 2 is a partially cutaway plan view illustrating a positional relationship between the lead frame and a lower mold member. Further, FIG. 3 is a cross-sectional view illustrating a state in which the lead frame is sandwiched by an upper mold member and the lower mold member, corresponding to the cross-section taken along lines III--III of FIG. 2.
The mold comprises an upper mold member 30 and a lower mold member 20, these mold members clamping the lead frame 10 in a sandwiching manner. Thus, a cavity 23 can be formed enveloping the island portion 3, on which the semiconductor chip is attached, and the inner lead portion. That is, a recess portion 25 is formed at the upper surface of the lower mold member 20 and, corresponding thereto, a recess portion 35 is formed at the lower surface of the upper mold member 30, these two recess portions 25, 35 cooperating to form the cavity 23.
At the upper surface of the lower mold member 20, a gate 21 and a runner 22, which are each adapted to inject the resin into the cavity 23, are formed. The gate 21 is provided at a position corresponding to one corner portion of the island portion 3. From the gate the resin may be injected into the cavity 23 in the direction of diagonal line of the square form of the island portion. At a position opposite to the gate 21, an air vent 24 is provided to evacuate air within the cavity as the resin is injected.
Injecting a synthetic sealing resin through the runner 22 and gate 21 into the cavity 23 causes air remaining within the cavity to be evacuated through the air vent due to the action of the injected resin. Thus, the resin is filled within the cavity 23 and, subsequently, by heating the mold, it is hardened. Finally, the mold are open, the tie bar 8 is cut away, the outer lead portion 8 and the suspension lead 7 are cut apart from the frame portion 5 and the resin burrs are removed to obtain the plastic package type, i.e. resin seal type, semiconductor device.
In the foregoing conventional lead frame, the interval between the outer peripheral portion of the island portion 3 and the inner end of the inner lead portion 4 is set substantially equal over the entire outer periphery. Therefore, when being injected from the gate 21 into a lower portion of the cavity 23, i.e. a portion lower than the lead frame 10, the molten resin will evenly flow from the lower portion toward an upper portion of the cavity over the entire periphery (as indicated by arrow in FIG. 3). The resin runs over the lead frame in such a manner of A.fwdarw.B.fwdarw.C.fwdarw.D.fwdarw.E as indicated in FIG. 2. That is, the resin first arrives at A, then B, then C, then D and then E. This is due to the fact that since the area between the semiconductor chip 2 and the upper mold member 30 is narrow, the flow rate of the resin is small, so that, in the neighborhood of the air vent the resin flowing from the lower portion toward the upper portion of the cavity flows back.
As a result, the air, which should originally be extruded by the injected resin, will remain involved at the portion overlying the semiconductor chip 2 within the injectd resin, which in turn will remain as air bubbles therein after hardened, and this residual air bubbles will degrade the reliability of the semiconductor device or its mechanical strength.